1. Field of the Invention
This invention relates to electroluminescent (EL) devices and more particularly to inorganic thin film EL devices which are utilizable as a plane light source or a display device.
2. Description of the Prior Art
Attention has now been paid to applications of inorganic thin film EL devices as a flat panel display device or a plane light source. For this purpose, there have been hitherto used inorganic fluorescent materials which comprise a matrix selected from at least one of ZnS, CaS and SrS, which is doped with not larger than 3 atomic percent of at least one element selected from Mn, Tb, Sn, Ce, Eu, Sm, Tm and the like as an emission center (Japanese Laid-open Patent Application No. 3-187191).
Of these, ZnS:Mn has been studied for use as an orange light emission fluorescent material and has been utilized as a flat panel display device because of its good characteristics such as luminance, life and the like. Moreover, zinc sulfides have been widely studied including ZnS:Tb used as a green light emission fluorescent material. Other types of fluorescent materials using alkaline earth metal sulfide matrices have also been studied extensively, including SrS:Ce as a blue light emission fluorescent material, CaS:Eu as a red light emission fluorescent material, CaS:Ce as a green light emission fluorescent material, and the like.
In view of the emission mechanism of fluorescent materials, with transition metals such as Mn, electrons of the matrix are directly collided with one another to emit a fluorescence. With alkaline earth metals, a large proportion of the light emission depends on the transition of an energy corresponding to the band gap of the matrix. The sulfides of alkaline earth metals have a band gap energy ranging from 4.3 to 4.4 eV and that of ZnS is 3.6 eV. In order to obtain blue to UV light emission requiring high energies, the band gap energy of these compounds is not so high. Thus, studies have been made on matrices having high band gap energies such as ZnF.sub.2 :Gd (7 to 8 eV), (J.J.A.P., Vol. 10B(1991), p. 1815-1816) and CaF.sub.2 :Eu (Appl. Phys. Lett., 41, 1982, p.462).
Inorganic fluorescent materials using the above-indicated sulfides other than ZnS:Mn are not satisfactory for use as a flat panel display device and a plane light source with respect to emission luminance, efficiency and life. In this sense, working color flat panel display devices have never been formed at present.